The present inVention relates to an apparatus for connecting a well on the ocean floor with a wellhead "Christmas" tree, (i.e., the flow control valves) on a fixed or relatively fixed platform, such as a floating tension leg platform or the like. More particularly, the present invention relates to an apparatus comprised of a riser tensioner system used in connecting the riser to the relatively fixed platform in order to avoid buckling of the riser. The tensioners of the present system apply a non-linearly responsive tension, the applied load increasing disproportionately at the back end in order to minimize the riser tensioner stroke length.
One of the benefits of a tension leg platform over other floating systems is the very small vertical oscillation that occurs. This enables the wellhead trees to be mounted within a few feet of a platform deck without the need for some complex form of motion compensation system. However, the use of a rigid riser system requires that a riser tensioner system be employed to compensate for the small amount of relative movement that does take place between the platform and the riser so that buckling or bending of the riser under its own weight will not result in a failure (cracking, breaking, etc.) of the riser. Heretofore, tensioner cylinders have typically provided a substantially linear load to the riser, i.e., that the tension load increases linearly in direct proportion to platform movement. Hence, the tensioner (both the cylinder and throw rod) must have a design length sufficient to accommodate the maximum platform movement possible (i.e., the movement caused by the design storm).
The present invention provides the desired motion compensation and tensioning of the riser by a plurality of tensioner cylinders which each have a non-linear response. That is, each tensioner provides a first rate of resistance (or tension) for normal platform movement and a second greater loading rate for storm-induced motion. This non-linear loading, in conjunction with the angulating of the riser tensioner cylinders such that they operate through a common point lying on the axis of the riser, enables the axial effective stroke length, and hence the distance between platform decks, to be significantly reduced. This can have an added benefit of reducing the profile of the floating platform and, hence, its wind loading, which reduces the forces that the tendons and risers will see and the size requirements for the already foreshortened riser tensioners.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following specification.